The present invention relates generally to near field optical microscopy (NSOM). More particularly, the invention relates to an improved fiber optic probe which provides dramatically improved efficiency and resolution.
The conventional compound microscope, now ubiquitous in the research laboratory, relies on illuminating the specimen by an external light source and using lenses in the far field to gather and focus the light. The far field corresponds to a specimen-lens distance of many optical wavelengths. There is, however, a limit to the resolving power of the conventional compound microscope. A physical phenomenon known as the diffraction limit prevents far field optical systems from resolving images smaller than roughly one-half the optical wavelength.
In 1928 Synge suggested that optical microscopy could overcome the diffraction limit of light by abandoning the far field and instead working in the near field. The near field exists in close proximity to the specimen, less than one optical wavelength. Using a tiny aperture and placing that aperture in the near field of the specimen, optical microscopy can achieve significantly greater resolving power. According to Synge's suggestion, the specimen is placed in the near field of the aperture, and light is delivered through the aperture so that it impinges upon and is reflected from or transmitted through the specimen. The reflected or transmitted light is then collected and viewed with a conventional optical microscope. In this way, only a tiny portion of the specimen is illuminated, hence all of the light entering the optical microscope corresponds to a single microscopic feature. This technique is thus capable of producing higher resolution than conventional optical microscopes.
A number of different implementations of Synge's idea have been experimented with. Pohl suggested that optical implementations could be achieved by coating the tip of a prism-like crystal. The first successful optical near field demonstration was by a group at Cornell which "taffy-pulled" glass micropipettes down to sub-wavelength diameters and defined the aperture by metallic overcoats. The taffy-pulled micropipette was highly inefficient, because the sub-wavelength diameter of the pipette choked off virtually all of the light, so that very little light would exit through the aperture to impinge upon the specimen.
Betzig and coworkers at AT&T Bell labs improved upon the Cornell taffy-pulled micropipette by replacing the glass micropipette with a fiber optic cable. Using the fiber optic cable Betzig and coworkers increased efficiency by three or four orders of magnitude. The Betzig device is manufactured by heating the fiber optic cable and then taffy-pulling it to sub-wavelength diameter, followed by a metallic overcoat.
While the Betzig device improves efficiency, a fundamental problem still remains. Although light will propagate efficiently down a fiber optic cable of standard diameter, the light becomes choked off when the diameter is reduced beyond a certain dimension. This is because light propagates in a waveguide-like fashion in the fiber optic cable of standard diameter. Specifically, light is confined to the inner core of the fiber optic cable by total internal reflection at the inner core-outer cladding boundary. When confined to the inner core in this fashion, light is said to be in the propagating mode.
However, when the diameter of the inner core is reduced, the propagating mode gives way to an evanescent mode. In the evanescent mode the optical energy is no longer truly propagating and is no longer confined to the fiber optic core, but rather a portion of the energy dissipates or escapes. The longer distance light must travel in this evanescent mode, the more energy that escapes.
It is a physical consequence of the taffy-pulling technique that the stretched fiber optic cable gets to be quite long before the diameter of the inner core becomes reduced sufficiently to form the aperture. Thus when a fiber optic cable is taffy-pulled to an extremely small aperture, the evanescent mode region is very long and efficiency is very poor. For example, a near field optical microscope (NSOM) with a resolution of 1000 .ANG. has an efficiency at best of roughly 2.times.10.sup.-4 ; by comparison, a NSOM with a resolution of 250 .ANG. has an efficiency of typically less than 1.times.10.sup.-6. The resolution plummets even further for smaller resolutions.
This degradation in resolution has significant consequences. Although extremely small apertures can be produced by taffy-pulling, the resulting efficiency is so low that virtually no usable light reaches the aperture and the specimen is not illuminated brightly enough to obtain a useful image.
The present invention overcomes the efficiency degradation problem by providing a light-emitting probe which has a rapidly tapered tip that protrudes longitudinally outwardly from the outer cladding of the fiber optic cable. The tapered tip comprises a portion of the inner core of the fiber optic cable. By extending the inner core longitudinally outwardly from the outer cladding, a very rapid taper can be fabricated by wet chemical etching. Because the inner core is rapidly tapered, light propagating along the inner core spends very little time in the evanescent mode before reaching the aperture. Therefore substantially more optical energy is delivered through the aperture to impinge upon the specimen.
Although other angular tapers may exhibit benefits of the invention, the presently preferred tip tapers at an acute angle on the order of about 15.degree. to 60.degree..
There are numerous applications where the high efficiency optical probe of the invention will be invaluable. These include, materials characterization, super high density magneto-optical memory and optical lithography. In addition, with the enhancement described next below, the high efficiency optical probe can be modified to yield extremely high resolutions never before attained. With this high resolution enhancement, rapid optical DNA sequencing is made possible. It is expected that optical DNA sequencing will provide a thousand-fold decrease in sequencing time, as compared to conventional electrophoresis techniques.
The high efficiency optical probe is enhanced by applying a metallic overcoat to the tapered tip and then supercooling (e.g. using liquid helium or liquid nitrogen). The supercooled metallic overcoat is thus rendered highly conductive and able to confine the optical energy to a very small aperture. To prevent thermal creep the specimen may also be supercooled in this fashion.
When extremely high resolution is desired an alternate embodiment of the invention employs a dual portion tip comprising a reduced diameter intermediate region and a pointed tip that extends longitudinally beyond the outer cladding of the intermediate region. The intermediate region may be formed with a gradual taper, as by taffy-pulling, and the pointed tip may be formed by chemical etching. Significantly, the intermediate region is configured with its gradually tapering diameter so that light propagating in the intermediate region does so in a guided wave mode. Light propagating in the pointed tip propagates in an evanescent mode, preferably for fewer than five optical wavelengths. In essence, the pointed tip employs the principles of the invention through a geometry that is able to provide a much sharper point. This is because the tapered tip begins at a reduced diameter, due to the diameter reduction afforded by the tapered intermediate region. Starting at a smaller diameter, the etching process produces a sharper point than it does when starting with larger diameters.
For a more complete understanding of the invention in both the high efficiency and enhanced high efficiency, high resolution forms, reference may be had to the following specification and to the accompanying drawings .